Technical Field
The present disclosure relates to methods for removing sulfur compounds from a fuel containing sulfur compounds using an adsorbent. The adsorbent comprises a carbonaceous material doped with nanoparticles of aluminum oxide. The carbonaceous material is at least one selected from the group consisting of activated carbon (AC), carbon nanotubes (CNTs), and graphene oxide (GO), and the adsorbent has a weight ratio of carbon (C) to aluminum (Al) in the range from 3:1 to 30:1, and a weight ratio of carbon (C) to oxygen (O) in the range from 1:1 to 10:1.
Description of the Related Art
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, is neither expressly nor impliedly admitted as prior art against the present invention.
Fossil fuels contain sulfur compounds. In jet fuel, the sulfur compounds include thiophene, benzothiophene (BT) and its derivatives. In marine gas oil, a naval logistic fuel, the sulfur compounds are mainly dibenzothiophene (DBT) and its derivatives. In diesel fuel, the major sulfur compounds are BT, alkyl-benzothiophene (alkyl-BT), DBT, and alkyl-dibenzothiophene (alkyl-DBT). The bulk of diesel fuel includes mainly saturated and aromatic hydrocarbons. Saturated hydrocarbons include n-paraffins, isoparaffins, and cycloparaffins (naphthenes). Aromatic compounds are mainly alky-benzenes, indanes, naphthalenes, tetralins, biphenyls, acenaphthenes, fluorines, acephenanthrenes, phenanthrenes, anthracenes, and naphthenophenanthrenes. Sulfur compounds comprising DBT and its derivatives are also present in used motor oil, since the sulfur compounds from fuels deposit on gas or diesel engines and increase wear of the engines.
In industrial and automobile waste gases, the sulfur compounds convert to SO2 and SO3 that produce acid rain. Additionally, the sulfur compounds in fossil fuels are adsorbed into catalytic converters and occupy the sites designed for CO, NO, and NO2 reduction, decreasing the reduction efficiency of and causing harm to the catalytic converters. As a result, removal of sulfur compounds from fossil fuels, particularly diesel fuel, is important for protecting the environment and eliminating the financial loss in products such as catalytic converters.
The European regulation required the sulfur content in diesel to be reduced from 2000 ppmw in 1993 to 50 ppmw in 2005 and to 10 ppmw in 2009. The U.S. Environmental Protection Agency required the sulfur content in highway diesel to be reduced from 500 ppmw to 15 ppmw in 2006. In 2012, nonroad diesel fuel used in locomotive and marine applications was required to meet the 15 ppmw standard.
Common desulfurization techniques for diesel include hydrodesulfurization (HDS), biodesulfurization (BDS), oxidative desulfurization (ODS), and adsorptive desulfurization. In HDS process, sulfur compounds in diesel are removed as hydrogen sulfide. This method utilizes hydrogen over a catalyst and applies high temperature up to 380° C. and high pressure between 500 to 700 psi. Aliphatic sulfides, thiols, thiophenes, and benzothiophenes (BTs) are easily removed because the sulfur atom in their molecular structure can access the active sites of the catalyst. Larger sulfur compounds such as dibenzothiophene (DBT) and alkyl-DBTs, particularly those with the alkyl groups at 4- and 6-positions, are much harder to remove in traditional HDS. These refractory sulfur compounds in diesel have difficulty in reaching the catalyst surface due to the steric hindrance caused by the carbon atoms bound to sulfur. Although improved deep HDS methods can overcome the problem to produce ultra low sulfur fuel, they require higher temperature, higher pressure, and more hydrogen and catalyst consumption, resulting in higher capital and operational costs. Additionally, the diesel treated by deep HDS has decreased lubricity that causes increased wear in a diesel engine.
Biodesulfurization (BDS) uses enzymes to remove the refractory sulfur compounds such as DBT and its derivatives under mild operating conditions through a pathway comprising two monooxygenases, which sequentially oxidize DBT to DBT sulfone and 2-hydroxybiphenyl-29-sulfinic acid, and a desulfinase, which converts 2-hydroxybiphenyl-29-sulfinic acid to the desulfurized end product 2-hydroxybiphenyl. In BDS, incomplete conversion of sulfur compounds occurs, resulting in the original substrate DBT and oxidized sulfur compounds (DBT sulfone and 2-hydroxybiphenyl-29-sulfinic acid) remaining in the fuel. The costs and stability of the biocatalysts in BDS are another obstacle for BDS to achieve commercial viability.
The oxidative desulfurization (ODS) method is another alternative for deep desulfurization of diesel to lower the temperature and pressure conditions and reduce the cost of operation. In this method, sulfur compounds in diesel, which are slightly more polar than their analogous hydrocarbons, are selectively oxidized to form their sulfoxides/sulfones that are highly polar in the presence of an oxidizing agent, most commonly H2O2, and a transition metal catalyst such as H3PM12O40 [M=Mo(VI), W(VI)]. The sulfoxides/sulfones can be subsequently extracted and removed by acetonitrile. However, extended reaction time to reach high yields, reaction safety due to high concentrations of H2O2, and its excessive decomposition are major impediments for commercializing this method.
Adsorptive desulfurization using solid adsorbents at lower temperature and pressure than hydrodesulfurization has been developed recently. ConocoPhillips Company introduced S-Zorb SRT for sulfur removal of diesel that uses a sorbent for attacking sulfur compounds. The sulfur atom remains in the sorbent but the hydrocarbon portion of the molecule is released. A stream of hydrogen in the process prevents the coke buildup. Another adsorption process called (PSU-SARS) was developed at Pennsylvania State University through selective adsorption at low temperature and ambient pressure without hydrogen consumption. Studies show that low sulfur results for different liquid fuels in this process have been achieved by using a composite metal catalyst on a porous substrate. This method will not adsorb the coexisting aromatic compounds like benzene and naphthalene.
Omid Etemadi investigated and proposed a desulfurization technique combining selective oxidation with adsorption using amorphous activated acidic alumina having a micrometer particle size or epoxy functionalized single wall carbon nanotubes (O-SWNT) to remove from oxidation-treated diesel benthiophene sulfone (BTO2) and dibenzothiophene sulfone (DBTO2), the oxidized products of BT and DBT, respectively (Etemadi, O., Selective adsorption in ultrasound assisted oxidative desulfurization process with nano-engineered adsorbents: Mechanism and Characterization (2007), incorporated herein in its entirety).
In order to meet increasingly rigorous emission control standards being imposed on fossil fuel products, effective, easy to use, and low cost desulfurization techniques, particularly for removing the refractory sulfur compounds from diesel, need to be developed to produce very low sulfur-containing or sulfur-free fuels.